(a) Field of the Invention
The present invention relates to an apparatus for OFDMA (Orthogonal Frequency Division Multiple Access) transmission and reception, and a method thereof. More specifically, the present invention relates to an apparatus for OFDMA transmission and reception for coherent detection in the uplink of a wireless communication system, and a method thereof.
(b) Description of the Related Art
The conventional OFDM systems include DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting) systems providing continuous bit streams for broadcasting services, or IEEE 802.11a and HIPERLAN/2 systems providing packet services in fixed or low speed mobile environment. In each system, reference symbols such as pilot symbols or preambles are designed to be suitable to a specific environment, and a channel estimation scheme adequate to the reference symbols is used, so as to achieve coherent detection.
There has recently been suggested a method combining OFDM and spread spectrum multiple access so as to support a higher data rate for multiple subscribers and achieve the frequency reuse factor of one in the cellular environment. The typical examples of the combined method are a method of using spread sequences (U.S. Pat. No. 6,188,717), and a frequency hopping OFDMA method using a frequency hopping pattern that is different for each cell (U.S. Pat. No. 6,473,418).
The cited patent, U.S. Pat. No. 6,188,717 (Feb. 13, 2001) discloses “A method of simultaneous radio transmission of digital data between a plurality of subscriber stations and a base station”, which is an OFDM-based multicarrier transmission and reception method for multiple subscriber stations that is applicable to both uplink and downlink of mobile communication systems. According to the cited patent, subscriber data are spread with orthogonal sequences and the spread data are overlapped and transmitted on allocated subcarriers. Here, there is no intersection of the subcarrier sets allocated to the respective subscriber stations and the allocated subcarriers are distributed in the whole band, so the channel estimation at the reception party is performed with a Wiener filter using reference symbols and the data are restored with a maximum likelihood sequence detector.
On the other hand, U.S. Pat. No. 6,473,418 (Oct. 29, 2002) discloses “An orthogonal frequency division multiplexing based spread spectrum multiple access”. To provide an OFDM-based cellular system for the reduction of the intercell interference, the cited invention proposes a system combining OFDM and frequency hopping based multiple access for cellular uplink and downlink multiple access so as to provide a frequency reuse factor approximate to “1” without a complex cell planning.
Both the systems provide a multiple access method applicable to the uplink as well as the downlink.
The method of using spread sequences, where the subcarrier set allocated to subscriber stations is fixed, enables channel estimation with a time domain filter using periodically inserted pilot symbols. However, the method demands the use of an interference reduction method such as maximum likelihood sequence estimation due to the intercode interference during signal detection, and requires transmission of pilot symbols on the whole subcarriers allocated to each subscriber station in the uplink.
The frequency hopping OFDMA method, where the subscriber stations in the same cell share pilots in the downlink, enables channel estimation for the whole frequency band in the downlink by using a pilot structure and a channel estimation method applied to the conventional circuit mode OFDM systems such as DAB, DVB, etc.
In the uplink, each subscriber station is required to use different reference symbols or pilot symbols for independent channel estimation, and the method demands frequency hopping in the unit of a block comprising adjacent time/frequency resources correlated in their channel responses so as to reduce the overhead caused by the pilot symbols. But, the intercell interference averaging performance can be degraded when the frequency hopping unit is too large. Apart from the frequency hopping system, the OFDMA system that allocates resources for uniform distribution of multiple user data in the whole frequency band so as to achieve frequency diversity is also required to allocate resources in the same manner as the frequency hopping OFDMA system.
The most traditional method for acquiring channel information in the uplink is allocating subcarriers such as consecutive OFDM symbols to the subscribers and performing differential coding and detection in the time domain. But, this method also requires the use of a reference symbol, causes a 3 to 4 dB performance loss relative to coherent detection in uncoded cases, and results in an increased performance loss in coded cases.
In a method of inserting pilots in the resource block and using pilot symbols to perform channel estimation, a performance better than differential detection can be achieved only by increasing the pilot insertion rate and boosting the pilot transmission power. However, the increased pilot insertion rate causes a deterioration of the spectral efficiency and the boosted pilot transmission power increases the intercell interference as well as the power consumption in the subscriber equipments.